The present invention generally concerns the monitoring of a patient's tissue using a photoplethysmographic device to generate information related to the concentration of oxyhemoglobin (O2Hb), deoxyhemoglobin or reduced hemoglobin (RHb), carboxyhemoglobin (COHb) and, additionally, methemoglobin (MetHb) in the patient's blood. More specifically, the application relates to a device for the measurement of such blood analytes using a set of emitters having optimal spectral contents defined by a set of specific wavelength ranges.
During emergency evaluation, surgery and other medical procedures, clinicians often want to know the oxygen concentration of the blood, as well as other factors. In pulse oximetry the relative concentration of oxyhemoglobin and deoxyhemoglobin is displayed as a percentage of total hemoglobin in order to provide data on the oxygenation of the patient's blood. Blood oxygenation can be adversely affected by the generation of additional hemoglobin species, called dyshemoglobins. Most notably, carboxyhemoglobin is generated when carbon monoxide molecules bind to the hemoglobin in blood. An accurate measurement of carboxyhemoglobin concentration in the patient's blood may be needed when the patient is a smoker or if carbon monoxide poisoning is suspected. Additionally, elevated methemoglobin levels in the blood can be caused by various medications, illegal drugs and certain pathological conditions such as sickle cell anemia. Therefore measurement of the methemoglobin concentration is also useful during patient evaluation.
Pulse-oximeters have been commercially available for measuring the oxygen saturation, or percent oxyhemoglobin, in arterial blood. These instruments rely on time-varying absorption of light by tissue supplied with pulsating arterial blood with a technique known generally as photoplethysmography. Conventional pulse oximeter instruments transmit light having spectral contents defined by two different center wavelengths through tissue.
The basic physical property that allows the measurement of arterial oxygen saturation by pulse oximetry is that the blood changes color with saturation. A pulse oximeter measures the "color" of the arterial blood and correlates this "color" to a given oxygen saturation to be displayed. When blood is well oxygenated it does not absorb a great deal of red light but as it desaturates it absorbs more and more red light giving the blood a darker appearance. The opposite behavior occurs in the near infrared region (from about 810 nanometers to 1000 nanometers) where hemoglobin absorbs more light when saturated with oxygen than when desaturated. For this reason current pulse oximeters use two emitters, usually light emitting diodes (LED's), one generating light in the red region, having a spectral content usually centered around 660 nanometers, and one generating light in the near infrared region having a spectral content usually centered around 925 or 940 nanometers.
The most obvious limitation of pulse oximetry derives from the fact that it is only a two channel (two wavelength bands) system and therefore can only solve for two components in the blood. Pulse oximetry makes the assumption that only oxyhemoglobin and reduced hemoglobin are present in the arterial blood. Any additional chromophores that are present in arterial blood and which absorb light in the wavelength bands used by the instrument will lead to erroneous readings. Two such chromophores are carboxyhemoglobin and methemoglobin. In particular, if carboxyhemoglobin or methemoglobin is present in above normal levels, a conventional pulse oximeter will give falsely high readings for the arterial oxygen saturation. This is one of the most serious and potentially dangerous limitations of current pulse oximetry.
Prior art pulse oximeters have so far lacked a means to compensate for errors in the measurement of oxyhemoglobin and reduced hemoglobin due to the presence of carboxyhemoglobin and methemoglobin in the blood. The manufacture of non-invasive devices which measure the concentration of various blood analytes such as oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin has been attempted by others. However, there has been no successful low-cost commercial implementation of a photoplethysmographic monitor which is capable of accurately measuring all four blood analytes. The present invention overcomes this deficiency by providing a novel method and apparatus for generating accurate concentration estimates for either three or four blood analytes: oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin and, in the four analyte embodiment, methemoglobin.
U.S. Pat. No. 5,413,100 to Barthelemy et al. (the "Barthelemy patent") describes a photoplethysmographic device for measuring oxyhemoglobin, reduced hemoglobin, and carboxyhemoglobin using three laser diode emitters having spectral contents characterized by the center wavelengths of 660 nm, 750 nm and 940 nm. By using an emitter having a spectral content characterized by a center wavelength of 660 nm as the lowest of the three spectral contents, a photoplethysmographic monitor designed according to Barthelemy et al. cannot accurately measure carboxyhemoglobin. No commercially acceptable photoplethysmographic device for the measurement of oxyhemoglobin, carboxyhemoglobin and deoxyhemoglobin has been developed based on the use of light detected from emitters having spectral contents as defined in the Barthelemy patent.